Loudspeakers



' LOUDSPEAKERS Filed Jan. 3. 1955 A. ATTOkNEYi United States Patent C)LOUDSPEAKERS 1 Donald Maynard Chave, Bromley, England ApplicationJanuary 3, 1955, Serial No. 479,624

Claimspriority, application Great Britain January 5, 1954 7 Claims. (Cl.181-31) This invention relates to loudspeakers and other soundreprodncers, and particularly to those which are required to radiatesound waves whose wavelength is of the order of dimensions of the movingparts of the reproducer, for example reproducers for treble sounds inmusic.

Reproducers have been made and described in which a conical movingmember such as a diaphragm is caused to vibrate along the axis of thecone, andalso in which a second smaller conical moving sound emitter oflesser vertical angle is attached to the first moving member. I havefound that the response of such a system to a range of high frequenciesis apt to vary undesirably with frequency because standing circularwaves are set up at certain frequencies in the material of the soundemitter, the mouth of which vibrates about its normal circular or othershape. I have found that another cause of such variation is the settingup of standing longitudinal waves in the material of the emitter.Variations of response due to the second cause are especially liable tooccur with the double-cone type of reproducer, whose inner cone oremitter has normally a completely free month, which thus presents adiscontinuity to longitudinal vibrations.

It is anlobject of the present invention to provide an improvedconstruction for sound reprodncers whereby these disadvantages areeliminated or reduced,.and the radiation of the highest frequency soundsrequired from a given reproducer is made more eificient and moreuniform.

According to the invention a sound reproducer has in combination'amovingsound emitter which is mounted to vibrate along anjaxis and haswalls which extend longitudinally of said axis, and a fixed core memberalso extending along said axis for at least the whole axial length ofthe emitter, the maximum spacing of the core member from theeinitter atany.cross section of the core member'perpendicular to the axis beingless than one ninth the outer iperipheryof the core .member at thatsection. Thus the reproducerrnay comprise a. moving sound emitterwhichfismounted to vibrate along an axis and has a circumfe'rentially'continuous wall which extends'longitudinally. of said axis, and a fixedcore member also extending alon said axis for at least the whole axiallength of the emitter, one end of the core mm L) projecting beyond theedge of the moving sound chants and being convexly rounded off, themaximum spacing of the core member from the emitter at any cross sectionof the core member perpendicular to the axis being led, than one ninththe outer periphery of the core member at that section.

According to another aspect of the invention a sound reproducercomprises a magnet system defining a gap across which magnetic flux isconcentrated, an electric inductor mounted to move within said gap underthe elfects of current in the inductor, a moving sound emitter in theform of a funnel operatively coupled to move as one with saidinductor,-:and a'fixedcore. member 2,808,895 Patented Oct. 8, 1957 2which is disposed inside the sound emitter and is attached to the magnetsystem, the fixed core member extending for the full axial length of thesaid sound emitter.

Sound reprodncers to which my invention is applicable are usuallyconstructed with conical 0r trumpet-shaped moving sound emitters, Whosecross-sections have a circular periphery, but my invention is equallyapplicable to cases in which the moving member is an elliptical cone, apyramid, or to other shapes.

I have found that the occurrence of circular standing waves is reducedwhen the compliance per unit area of the air between the fixed andmoving members is reduced by reducing the gap, and that this gap mayadvantageously be substantially less than the amount of one ninth (orpreferably one tenth) of the periphery quoted as an effective limit.

I have also found that longitudinal standing waves may be minimised bycontinuing the fixed member along the axis, for example by terminatingit with a hemispherical or other convex rounded end.

In certain constructions further improvements may be made by providingthe fixed member with blind or communicating holes. It may be made ofsound-reflectant or sound-absorbent material, such as plaster of Paris.

In addition to the advantages herebefore described, the improvedarrangement distributes the radiation of high-frequency sound (forexample, frequencies above 4000 cycles per second) more uniformly overthe azimuthal range from the axis of the loudspeaker.

The invention is illustrated by way of example in the accompanyingdiagrammatic drawings, in which:

Figure l is a fragmentary sectional elevation of the front of animproved sound reproducer;

Figure 2 is a transverse sectional elevation taken on the line 2-2 ofFigure 1 and drawn to a reduced scale;

Figure 3 is a diagram showing the improved reproducer installed in abaffle board;

Figure 4 is a diagram of an improved reproducer feeding a horn;

Figure 5 is a diagrammatic section of a modified construction; and

Figure 6 shows the reproducer feeding a horn substantially at rightangles.

The reproducershown in Figures 1 and 2 is of the moving coil type andhas a magnet system which is indicated generally at It} including acentral pole piece 11 disposed within the aperture of an outer polepiece 12 to form an annular gap 13 across which there is an intensemagneticfield. A tubularformer-14 is mounted so as to extend into thegap 13 with freedom to oscillate within narrow limits along an axiswhich is indicated at 15, and it carries an inductor in the form of acoil of insulated wire 16,-the mounting (not shown) being arranged, asis well known in the art, so that neither the former 14 nor the coil 16will touch either of the pole pieces 11, 12, despite the narrowness ofthe gap 13. When an alternating current is passed through the co 16 theinteraction of the produced inzvgnetic Nitil the flux crossingthe gap13, of course causes the coil 5 to reciprocate along the axis 15 andthus drive the former 14 correspondingly. Attached to the front end ofthe former 14 is a frusto-conical diaphragm 17 which has its rimconnected to an annular supporting frame 18 by a flexible surround 24}adapted to allow the diaphragm 17 to reciprocate with the former 14.

With a View to improving the emission of sounds in the higher frequencyrange (say. 1000 cycles per second and higher) a frusto-conical emittermember 21 is fixedly secured to the former 14 or to the centre of thedia phragm 17 so as to extend forwards truly concentric with the axis15, its free .edge yZZ conveniently being 3 approximately co-planar withthe periphery of the diaphargm 17.

In order to stabilise the emitter member 21 against unwanted modes ofvibration, and also to improve the distribution of sound from saidemitter member, a central core member 23 is rigidly mountedupon thecentre pole piece 11, say by means of an axial stud 24 threaded into thepole piece and passing snugly through a bore 25 in the core member, thelatter being held in position by a nut 26. The core member 23, which ispreferably made of plaster, such as plaster of Paris, has a cylindricalnecl: 27 adjoining the pole piece 11, and its outer surface thendiverges in the forward direction, the profile being curved, asindicated at 28, until the position of maximum diameter is reached at30, this being just forward of the rim 22 of the emitter member 21. Fromthis point the front of the core member is rounded off to form asubstantially hemispherical nose 31. The core member 23 must extend forthe whole axial length of the emitter 21 in order to produce the desiredcontrol over the vibrations of the emitter, and it is important that itsouter surface should lie relatively closely to the inner surface of theemitter. Experiments have shown that the maximum distance from theoutside of the core member 23 (at any given cross section perpendicularto the axis to the inside surface of the emitter 21 should be less thanone ninth the length of the periphery of the core member at thatsection. Probably one tenth is the working limit. Thus taking point 32on the core member at the cross section denoted by the line 2-2 (whichis of course at right angles to the axis 15) the nearest distance d tothe inside surface of the emitter member 21, when the latter is in itsnatural position, must be less than one ninth of the periphery p of thecore member 23 at that section, as shown in Figure 2. This applies inall cases, irrespective of whether the core member is concentric withthe emitter member, and for different cross sectional shapes of emittermembers and/or core members. In Figure 1 the ratio of d/ p isapproximately 1/20 at the plane 2-2 and is about 1/16 at the plane ofthe free edge 22.

It will be seen in Figure 1, that there is an annular passage 33 formedbetween the core member 23 and the emitter member 21, and that thisincreases in cross sectional area as it progressively gets larger indiameter in the forward direction away from the former 14. Indeed theinner surface of the emitter member 21 diverges at a greater rate thanthe outside of the core member so that the passage 33 increases inradial width, the curved profile 28 of the core member, enabling thepassage 33 to be designed to have specific sound propagatingcharacteristics in the nature of a horn.

The improved form of sound reproducer may be modified in construction tosuit the use for which it is required. Thus a unit similar to that shownin Figures 1 and 2 may be provided with a bafile board and will radiatea wide band of frequencies covering practically the whole of the audiblerange, the lower limit being dependent on the size of the bafiie board.This arrangement is shown in Figure 3, the baflle board being attachedto the frame 18 and being indicated at 34.

In Figure 4 the emitter member 21 is provided with a flexible surround21a by which it is connected to the entry of a horn 35, the core member23 being rigidly mounted upon the magnet assembly 10 so as to extendaxially within the fnlsto-conical emitter member 21 and control the modeof vibration thereof.

If desired a plurality of emitter members may be arranged one within theother, the core member being of course disposed within the inner one asis shown in Figure 5. The construction is the same as that in Figure land the corresponding parts are indicated by the same referencenumerals. One additional emitter member 36 is disposed between thediaphragrn 17 and the emitter member 21 and is conveniently attachedrigidly to the coil former 14, its front edge 37 being free.

In some cases, such as in sound reproducers of the form described in myco-pending application Serial No. 479,599, filed January 3, 1955, it maybe desirable to have the axis 15 of the reproducer unit disposed at anangle to the length of a horn or like sound delivery device, and thearrangement shown in Figure 6 may advantageously be used. A horn 38whose axis is shown at 40 has its lower wall curved upwards at 41 tomeet a horizontal inlet aperture 42 over which the sound reproducer isfixed with its axis 15 substantially vertical, said reproducer beingindicated generally at 43 and being of the same construction as that inFigures 1 and 2. In order that the higher frequency vibrations from theemitter member 21 may be more efficiently delivered along the axis 40 ofthe horn 38, a plate 44 is fastened in an oblique position upon the coremember 23, and this acts in the manner of a reflector for sound wavespassing downwards from the emitter member 21.

It is found that with the core member in position, the sounds of higherfrequency are more uniformly dispersed, instead of tending toconcentrate on the axis of the reproducer as at present, and moreoverthe usual cancellation effects at the edges and sides of the emitter arelargely avoided. This applies especially to the reproduction offrequencies of over 4000 cycles per second. Moreover the core memberoffers an air loading to the emitter member, and also defines a shortfunnel or horn (of an nular cross section) through which air vibrationsfrom the coil itself are transmitted to the front of the loudspeaker.The radial width of this funnel or horn space is substantially less thanthe wavelength of the sounds which are to be reproduced therethrough.

The above details are given by way of example only and it will beapparent that various modifications may be made to suit requirements.

What I claim is:

1. A sound reproducer comprising a sound emitter member in the form of ahollow frusto-cone having a rim portion and a central root portion,driving means at the root portion for vibrating the emitter member alongits axis, a fixed support member adjacent the root portion of theemitter member, and a rigid core member which is fixedly mounted uponthe support member and extends inside said sound emitter member, whichcore member comprises a divergent portion extending from the supportmember for the full axial length of the emitter member and a frontportion which is wholly in front of the rim portion of the emittermember and is of domed shape.

2. A sound reproducer comprising a sound emitter member in the form of ahollow frusto-cone having a rim portion and a central root portion,driving means at the root portion for vibrating the emitter member alongits axis, a fixed support member adjacent the root portion of theemitter member, and a rigid core member which is fixedly mounted uponthe support member and extends inside said sound emitter member, whichcore member comprises a divergent portion extending from the supportmember for the full axial length of the emitter member and a frontportion which is wholly in front of the rim portion of the emittermember and is a substantially hemispherical shape.

3. A sound reproducer comprising a sound emitter member in the form of ahollow frusto-cone having a central root portion by which the emittermember is supported, and a rim portion which is free, driving means atthe root portion for vibrating the emitter member along its axis, afixed support member adjacent the root portion of the emitter member,and a rigid core member which is fixedly mounted upon the support memberand extends inside said sound emitter member, which core membercomprises a divergent portion extending from the support member for thefull axial length of the emitter member and a front portion which iswholly in front of the rim portion of the emitter member and is of domedshape.

4. A sound reproducer comprising a sound emitter member in the form of ahollow frusto-cone having a central root portion by which the emittermember is supported, and a rim portion which is free, driving means atthe root portion for vibrating the emitter member along its axis, afixed support adjacent the root portion of the emitter member, and arigid core member which is fixedly mounted upon the support member andextends inside said sound emitter member, which core member comprises adivergent portion extending from the support member for the full axiallength of the emitter member and a front portion which is wholly infront of the rim portion of the emitter member and is of substantiallyhemispherical shape.

5. A sound reproducer comprising a sound emitter member in the form of ahollow frusto-cone having a rim portion and a central root portion,driving means at the root portion for vibrating the emitter member alongits axis, a fixed support member adjacent the root portion of theemitter member, and a rigid core member which is fixedly mounted uponthe support member and extends inside said sound emitter member, whichcore member comprises a divergent portion extending from the supportmember for the full axial length of the emitter member and a frontportion which is wholly in front of the rim portion of the emittermember and is of substantially hemispherical shape, the distance ofevery point on the said divergent portion of the core member from thenearest point on the interior of the emitter member being less thanone-ninth of the perimeter of the core member at the said point.

6. A sound reproducer comprising a sound emitter member constituted by aflexibly mounted diaphragm and a hollow frusto-cone having a centralroot portion secured to the diaphragm and a free outer rim portion,driving means at the root portion for vibrating the emitter member alongits axis, a fixed support member adjacent the root portion of theemitter member, and a rigid core member which is fixedly mounted uponthe support member and extends inside said sound emitter member, whichcore member comprises a divergent portion extending from the supportmember for the full axial length of the emitter member and a frontportion which is wholly in front of the rim portion of the emittermember and is of domed shape.

7. A sound reproducer comprising a sound emitter member constituted by aflexibly mounted diaphragm and a hollow frusto-cone having a centralroot portion secured to the diaphragm and a free outer rim portion,driving means at the root portion for vibrating the emitter met heralong its axis, a fixed support member adjacent the root portion of theemitter member, and a rigid core member which is fixedly mounted uponthe support member and extends inside said sound emitter member, whichcore member comprises a divergent portion extending from the supportmember for the full axial length of the emitter member and a frontportion which is wholly in front of the rim portion of the emittermember and is of substantially hemispherical shape, the whole of saidfront portion being in front of the said diaphragm.

References Cited in the file of this patent UNITED STATES PATENTS

